Chapter 12: The Auditory and Vestibular Pathways and Approach to Hearing Loss and Dizziness/Vertigo: Cranial nerve 8

INTRODUCTION

Cranial nerve 8 (CN 8) contains two components: auditory (cochlear) and vestibular. Both begin in the inner ear and travel to the brainstem: the auditory component projects to the cochlear nuclei (at the pontomedullary junction) and the vestibular component projects to the vestibular nuclei (in the medulla).

THE AUDITORY SYSTEM

Auditory information travels from the inner ear through the auditory (cochlear) portion of CN 8 to arrive at the cochlear nuclei at the pontomedullary junction (Fig. 12–1). The cochlear nuclei project to the inferior colliculi of the lower midbrain via the lateral lemniscus, and also project to the superior olives. Each inferior colliculus projects to the ipsilateral medial geniculate nucleus (MGN) of the thalamus, and each MGN projects to the ipsilateral auditory cortex in the superior temporal gyrus (Heschel’s gyrus).

APPROACH TO HEARING LOSS

Auditory information crosses to become bilateral early in its connections within the brainstem, so unilateral hearing loss can only occur due to pathology of the inner ear or CN 8 (or rarely the entry zone of CN 8 or cochlear nuclei at the pontomedullary junction). Central lesions (in the brainstem or temporal lobe) only rarely cause deafness, and must be extensive and bilateral to do so. Therefore, central etiologies of deafness are usually associated with other signs due to involvement of neighboring structures. Left temporal lobe lesions can lead to deficits in word processing (pure word deafness) and right temporal lobe lesions can cause deficits in music processing (amusia).

Hearing loss due to a peripheral lesion is called conductive hearing loss if it caused by problems in the outer or middle ear, and called sensorineural hearing loss if it is due to problems in the cochlea or auditory component of CN 8. Both conductive and sensorineural etiologies of hearing loss may be acquired or may have a congenital/genetic basis. Acquired causes of hearing loss are listed below.

Acquired causes of conductive hearing loss include:

• Outer ear: cerumen impaction

• Middle ear: infection (otitis media), otosclerosis

Acquired causes of sensorineural hearing loss include:

• Unilateral

  • Internal auditory artery infarct (the internal auditory artery [also called the labyrinthine artery] is usually a branch of the anterior inferior cerebellar artery [AICA])

  • Sudden sensorineural hearing loss (often idiopathic; may respond to steroids)

  • Ménière’s disease (see “Ménière’s Disease” below)

  • Vestibular schwannoma (also called acoustic neuroma; see “Vestibular Schwannoma” in Ch. 24)

• Bilateral

  • Aging (presbyacusis)

  • Ototoxic medications (e.g., aminoglycosides)

  • Sequela of meningitis (especially in children)

  • Neurofibromatosis type II with bilateral vestibular schwannomas (see “Neurocutaneous Syndromes” ...